In C. elegans males, laterally located V cells generate a simple pattern of anterior alae (cuticular ridges) and posterior rays (mating sensilla). We have found that this pattern is generated, at least in part, by the selective interruption of cell-cell interactions. In anterior V cells, lineages leading to the production of alae are induced by cell interactions. These cell interactions are inhibited in specific posterior V cells by the activity of the gene pal-1, which allows these cells to generate rays instead of alae. The activities of cell signals and pal-1 appear to influence V cell fates by determining the state of a developmental switch that involves two homeotic genes, lin-22 and mab-5.